This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-185848, filed Jun. 30, 1999, the entire contents of which are incorporated herein by reference.
This invention relates to solid-state image device which is used in CCD (Charge Coupled Device) and CMOS image sensors.
A cause of noise included in signals transmitted from a solid-state image device is the surface recombination phenomenon observed on the surface of the photodiode.
FIG. 8 is a schematic diagram illustrating the structure of a CMOS image sensor of conventional art used in a solid-state image device. The CMOS image sensor has in principle a photodiode 107 and a gate electrode 105. The photodiode 107 receives photons, converts them into electric signals, and accumulates electric charge. The gate electrode 105 is used to read out the charge accumulated in the photodiode 107.
In this CMOS image sensor, a P-well region 102 is formed on a P-type semiconductor substrate 101, for example. An element isolation film 103 is selectively formed on the surface of this P-well region 102. within the region confined by the element isolation film 103, a gate oxide film 104 is formed on the surface of the P-well region 102. On the gate oxide film 104 and almost in the center of the region confined by the element isolation film 103, the gate electrode 105 is formed. In the channel region in the surface of the P-well region 102, impurity is introduced to provide an impurity layer 106. The impurity layer 106 controls the threshold voltage of a MOS transistor containing the gate electrode 105.
In the P-well region 102 between the element isolation film 103 and the gate electrode 105, a photodiode layer 107 is formed. The photodiode layer 107 is formed by implanting an N-type impurity into the P-well region 102. T he photodiode layer 107 is formed in self-alignment with the readout gate 105. o n the surface of the photodiode layer 107, a surface shield layer (P+) 108 is formed. The surface shield layer 108 is formed by ion implanting a high concentration of a P-type impurity into the P-well region 102. This surface shield layer 108 works to prevent depletion of the surface of the photodiode layer 107 and formed in self-alignment with the gate electrode 105.
In the P-well region 102 to the opposite side of the surface shield layer 108 beyond the gate electrode 105, a drain region 109 is formed. When a voltage is applied to the gate electrode 105, the charge read out from the photodiode layer 107 is transferred to the drain region 109. The drain region 109 is formed in self-alignment with the readout gate 105. Part of the gate oxide film 104, located on the drain region 109, is removed to form a drain electrode 110 there. The charge transferred to the drain region 109 is supplied to a detector (not shown) through the drain electrode 110.
In the conventional CMOS image sensor having a photodiode layer 107 and a gate electrode 105, surface recombination (generation of dark current) in the photodiode layer 107 is prevented by forming a surface shield layer 108 on the surface of the photodiode layer 107.
This type of CMOS image sensors, however, have a surface shield layer 108 that covers all the surface of the photodiode layer 107. As a result, a region 111, which works as a potential barrier, is formed near the gate electrode 105 in the surface shield layer 108 when a voltage is applied to the gate electrode 105 to read out the charge accumulated in the photodiode layer 107. This region 111 extends above the photodiode layer 107 toward the side surface of the channel region which is formed under the gate electrode 105. Then it becomes hard to read out charge because of this potential barrier 111 when the charge is read out from the photodiode layer 107. This phenomenon poses a problem that the gate electrode 105 requires a high voltage for reading out charge from the photodiode layer 107.
It is an object of the present invention to provide a solid-state image device and method of manufacturing the same that can prevent the generation of dark current and transfer charge in the charge accumulation layer with a low voltage.
According to a first aspect of the present invention, there is provided a solid-state image device comprising: a semiconductor layer of a first conductivity type; a charge accumulation layer of a second conductivity type formed in the semiconductor layer, the charge accumulation layer performing photoelectric conversion and accumulating charge; a gate electrode formed on an insulator film on the semiconductor layer and reading out charge in the charge accumulation layer, the gate electrode being positioned above part of the charge accumulation layer; a drain region of the second conductivity type formed in the surface of the semiconductor layer corresponding to one side of the gate electrode, to which charge read out from the charge accumulation layer is transferred by the gate electrode; a punch-through stopper region of the first second conductivity type formed between the drain region and the charge accumulation layer; and a shield layer of the first conductivity type formed on the surface of the semiconductor layer corresponding to another side of the gate electrode, the shield layer contacting a surface of the charge accumulation layer.
According to a second aspect of the present invention there is provided a solid-state image device comprising: a semiconductor layer of a first conductivity type; a gate electrode formed on an insulator film on the semiconductor layer; a charge accumulation layer of a second conductivity type formed to one side of the gate electrode, the charge accumulation layer performing photoelectric conversion and accumulating charge; a drain region of the second conductivity type formed in the surface of the semiconductor layer to another side of the gate electrode, to which charge read out from the charge accumulation layer is transferred via the gate electrode; and a shield layer of the first conductivity type formed in the surface of the semiconductor layer on the charge accumulation layer, wherein the position of the charge accumulation layer is shifted from the surface shield layer toward the gate electrode.
According to a third aspect of the present invention there is provided a method of manufacturing a solid-state image device comprising the steps of: forming a gate insulator film on the surface of a semiconductor layer of a first conductivity type; forming a charge accumulation layer by implanting a second conductivity type impurity into the semiconductor layer, the charge accumulation layer performing photoelectric conversion and accumulating charge; forming an impurity layer by implanting the first conductivity type impurity into the semiconductor layer to control the threshold voltage; forming a gate electrode on the gate insulator film, the position of the gate electrode being shifted toward one end of the charge accumulation layer; forming a punch-through stopper region of the first conductivity type in the semiconductor layer, in self-alignment with the gate electrode, the punch-through stopper region confining one end of the charge accumulation layer; forming a shield layer of the first conductivity type in the semiconductor layer, in self-alignment with the gate electrode, the shield layer contacting another end of the charge accumulation layer; and forming a drain region of the second conductivity type in the semiconductor layer, in self-alignment with the gate electrode, the drain region contacting the punch-through stopper region.
According to a fourth aspect of the present invention there is provided a method of manufacturing a solid-state image device comprising the steps of: forming a gate insulator film on the surface of a first conductivity type semiconductor layer; forming an impurity layer by implanting the first conductivity type impurity into the semiconductor layer to control the threshold voltage; forming a gate electrode on the gate insulator film; forming a drain region of a second conductivity type in the semiconductor layer, in self-alignment with the gate electrode; forming a charge accumulation layer of the second conductivity type in the semiconductor layer, in self-alignment with the gate electrode, the charge accumulation layer being formed to the opposite side of the drain region beyond the gate electrode, part of the charge accumulation layer extending to a position under the gate electrode; forming a sidewall film on the wall of the gate electrode; and forming a shield layer of the first conductivity type in the semiconductor layer, in self-alignment with the sidewall film, the shield layer being displaced from the charge accumulation layer.
According to fifth aspect of the present invention there is provided a method of manufacturing a solid-state image device comprising the steps of: forming a gate insulator film on the surface of a semiconductor layer of a first conductivity type; forming an impurity layer by implanting the first conductivity type impurity into the semiconductor layer to control the threshold voltage; forming a gate electrode on the gate insulator film; forming a drain region of a second conductivity type in the semiconductor layer, in self-alignment with the gate electrode; forming a sidewall film on the wall of the gate electrode; forming a shield layer of the first conductivity type in the semiconductor layer, in self-alignment with the sidewall film; removing the sidewall film; and forming a charge accumulation layer of the second conductivity type in the semiconductor layer, in self-alignment with the gate electrode, the charge accumulation layer being formed to the opposite side of the drain region beyond the gate electrode, part of the charge accumulation layer extending to a position under the gate electrode, the charge accumulation layer being displaced from the shield layer.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.